This invention relates generally to non-destructive evaluation of pipelines and more particularly to a method and apparatus for inspecting electrically conductive structures using pulsed eddy current.
Pipelines are widely used in a variety of industries, allowing a large amount of material to be transported from one place to another. A variety of fluids such as oil and/or gas are transported cheaply and efficiently using pipelines. Particulate matter, and other small solids suspended in fluids may also be transported through pipelines. Underground and underwater (deep sea) pipelines typically carry enormous quantities of oil and gas products that are important to energy-related industries, often under high pressure and at extreme temperatures and at high flow rates.
Flaws in constituent pipes may cause pipeline integrity degradation as the pipeline infrastructure ages. Corrosion of a pipeline can be caused by small spots of weakness, subsidence of the soil, local construction projects, seismic activity, weather, and simply wear and tear caused by normal use, and can lead to defects and anomalies in the pipeline. Thus, flaws or defects and anomalies can appear in the surface of the pipeline in the form of corrosion, mechanical damage, fatigue, crack, stress, corrosion cracks, hydrogen induced cracks, or distortion attributable to dents or wrinkles.
Maintaining and protecting existing pipeline networks is proving to be a challenge. Current state-of-art inline inspection systems employ devices known as pipeline inspection gages (PIGs) to traverse sections of pipe in situ and provide data that may be evaluated to identify structural defects. Such PIGs acquire data from multiple sensors while traveling inside the pipeline. A typical single run for the PIG may be more than 100 km long. The use of PIGs allows evaluation of the integrity of a pipeline section without costly excavation and insulation removal to get access to the outer wall and conduct nondestructive inspection of the pipeline section.
PIGs may employ a wide range of sensor technology to collect information about pipelines. Examples of technologies that may be used include magnetic flux leakage (MFL), ultrasound (UT) or eddy current (EC). Each of these methodologies has its limitations. For instance, MFL systems rely on high field permanent magnets, which are bulky, heavy and have significant dragging force. As a result, PIGs employing MFL technology are suitable for inspecting pipelines that have relatively smooth bends. The UT method requires mechanical coupling with pipe walls and is not suitable for gas pipes or contaminated walls. Existing EC pigs are typically employed to inspect non-magnetic metal piping. In carbon steel pipes, the depth of penetration is of eddy currents is relatively small because of magnetic permeability which leads to a low frequency solution using large inductive coils for deep penetration and large area integration to prevent local variations of magnetic permeability. The need for deep magnetic penetration and large area integration makes EC pigs not suitable for restrictive pipeline environments that have relatively sharp bends.
Remote field EC and transient EC technologies have been developed to overcome some of the aforementioned problems. However, remote field EC and transient EC technologies do not facilitate the inspection of large diameter, thick carbon steel pipelines with high spatial resolution to detect areas of pitting corrosion with a moving PIG. Since remote EC systems use a spatial separation between exciting and sensing elements, large areas adjacent to sharp turns and valves are left uninspected. Additionally, remote field EC and transient EC technologies do not facilitate low power consumption for automatic PIGs. A PIG adapted to facilitate internal inspection of pipelines that have sharp turns and valves with reduced clearance is desirable.